Sometimes, teamwork is essential. If you want to get 3 stars for all thieves, or just be nice to your fellow player.
Here are possible ways you could help your team:
Use Air-Support Gadgets. A Health or Shield Drop might save someone's life.
If one of your friends step on a bomb trap, disarm the bomb!
Use switches to move something in the environment, Ex: The switch on top of the crates on the "Docks" map.
If you are infected, don't go anywhere near your teammates.
Taunt the sniper and dodge the bullets so your teammates can move up, then go in at the same time to disorient the sniper.
If the Sniper's Eye is on someone else, take advantage of this and activate a trap.
Leave down Buddy Health/Stamina or Bud-renogain.
If you want to deactivate a trap, use the E.M.P Device. Since that will set off the traps, using Trap-Avoid will only get you yourself farther.