A character model may join a war machine's crew in the same way as he might join a unit of troops, by moving to within 1" of the machine itself. The war machine benefits from the associated character's Leadership value in the same way that a unit of troops would. However, only crew can operate a machine, a character cannot (unless specified in their special rules).
On the positive side, this means that characters are never hurt should the machine misfire. Characters are sensible or experienced enough to keep sufficient distance between themselves and the dangerous mechanism to be safe.
Characters are free to fire their own missile weapons at a different target from the one that the machine is being fired at.