Movement is an important part of combat. It can keep you out of harms way, it can allow you to take on massive odds otherwise not possible, or it can doom you to be ripped apart by a group of zombies. It's also one of the major decisions, along with your action, that you can make in a round of combat in Dungeons & Dragons 5th edition. However, keeping it interesting can be a bit of a challenge. Often players will come up with a formation and stick to it. For that reason, I hope to share some of my thoughts and twists for making movement play a bigger role. In retrospect, I really should have done this topic before confined spaces and many enemies. Alas, here we are.
Reason to Move
From the start, there needs to be a good reason to move. Standing still can be the right choice in some cases. As a fighter, you might want to protect your wizard from arrows. However, in these cases movement options still need to look attractive or there really isn't a choice being made. What often tends to happen, from my experience, is that combat will start and something will happen. Maybe the enemy is primarily melee based and they will try to charge forward. However, after that the fight locks in place and stays static until something dies. Again, fights can go this way, but I think it should be a choice made by the players instead of simply the default and best option.
A classic reason to need to move is that something will happen if you don't. Maybe you'll get charged at by a giant creature. Maybe you need to duck down and move so that the wizard's fireball has a chance of missing you. In these cases, there is often a need for cover to hide behind or something special about the creature in question that can be avoided through movement but not by standing still.
A goal can force different kinds of movement and formations. If the players are chasing after someone who has something they need, they will be forced out of a static formation. Likewise, if they need to get something in the hands of one particular bad guy, it gives an incentive to break formation and go after the objective. When being used, I find it's better when breaking formation is a tempting option but not the only option. Perhaps they can go from rooftop to rooftop trying to shoot them down with an arrow. Perhaps they can chase after them on the streets. They can also try to get into formation and throw insults at the bad guy, hoping it'll force them to move. In this last case, they'll still be in formation but other options will probably at least be thought of. It also still something the players thought of and tried to do.
There are plenty of things you can throw into the environment in order to make movement look tempting. Adding some form of verticality to the encounter can give new tactical options. However, to employ them the character needs to get into position first. You can also have your encounter take place in a collapsing tunnel, forcing a weird encounter where both sides are running away while still trying to off each other. Another option is to have cover that might get destroyed if it takes too much damage. One of my favourites is to include environmental hazards or traps. Things such as drops, holes in the floor, or actual traps that might be noticed in combat and exploited all setup interesting encounters. Maybe the players will fake a retreat in order to get their pursuers to trigger a trap they noticed when fighting. Maybe the enemy will.
A common reason for a static formation is that the enemies always come from one direction. This means that the players can just set up their formation, block the tunnel and slog through the rest. When multiple directions are used, things tend to get more interesting. Even in the worst case scenario, the formation will need to turn to face the new threat. This can still be a tense moment due to the order things happen (initiative order can make things tense in such situations), unnoticed threats or simply being unable to respond completely perfectly in time.
Why Standing Still Is Bad
If you want to have elements of combat that prevent movement, then it should be a real loss. If you prevent a character from moving when they'd just rather stay in formation anyway, they haven't really lost anything. For this reason I like to consider why I'm adding an encounter feature that prevents movement. If I can't think of one, I don't put it in. If players are moving to areas of light and getting caught by a shadow hand coming from the ground will allow the shadows to catch them, there is a reason to move. Of course players can attack the hands and set each other free. However, when they know they have to in order to survive and need to consider giving up their important action to regain their ability to go to safety, both options can look tempting.
Built into D&D 5th edition are multiple things that force movement for players. I've seen far too many times when a Dungeon Master forgot about the ability to shove creatures or grab them and drag them out of the way. This is especially true where zombies are concerned. However, there are other such effects too. From feats to spells such as Thunderwave, there are many ways that movement might be forced on players or creatures alike. However, we can also do it in other ways. I'd call falling a method of forced movement. However, being on a ship as it's tilting can also end up forcing movement when the angle is too steep. Flying in a storm can do so as well. However, when I do so I don't want to just take away the players ability to move. I find that it works better to force players to trade something in order to move, or still allow them to move but shift it a little. If the player tried to fly 15 feet to the left, the wind might shift their movement path but they still moved. This means moving and dodging things can still be done, but there is a risk associated with it. For this reason I thing being careful not to take the option away completely, particularly when often holding formation is the go to choice, is important.
You can also create special effects or conditions to increase movement. Getting double the movement speed or being in a special location where they can fly gives them new tactical options. Having these options lost to you makes the impact bigger.
Many of the things I mentioned earlier will come off as novelties. However, it can depend heavily on your type of campaign. If your campaign is more horror oriented and players will often need to avoid contact with things or flee, it can be a reoccurring feature of your combat encounters. The big ones that apply most of the time is the direction the players are being attack, stealth, goals, cover, and environmental hazards. The other cases are nice and make combat memorable but don't feature as often unless your campaign is based around them from my experience.