Fights often have onlookers. If a group of PCs gets into a brawl with some street ruffians, they will certainly attract a crowd of curious watchers. The authorities may arrive sooner or later, but until they do there will be various other people hanging around. And if modern-day behaviour is anything to go by, most of those will be involved in some variation of egging the fighters on or cheering for one side or the other.
You can adapt this to other locations and genres. Imagine trying to take out a guardroom of orcs, and having a large group of additional orcs charge in, and start cheering on their boys from the sidelines. It can be a good way to have more potential foes in the vicinity without completely overwhelming the PCs in the battle. If the orcs win, then the onlookers can cheer and hoot while the guards tie up the PCs as prisoners. If the PCs win, you have two choices.
First, the onlookers can take offence and charge into the fray, continuing the battle. But secondly, you can play it the other way. Have the onlookers look stunned that the PCs have prevailed, stare open-mouthed for a tense few seconds, then flee for the hills. By holding some of the forces back as onlookers, you leave these options open, depending on how challenging you want the overall encounter to be and how well the PCs' luck holds out in the combat.
Darth Vader: Your powers are weak, old man.
Obi-Wan: My robes may be moth-eaten, but you'll see I still have some fight left in me.
Darth Vader: I have discovered Force powers never dreamt of in your philosophy.
Obi-Wan: I know you have complete control over the clones because of their lack of midi-chlorians.
Obi-Wan: They are harmless without your malevolent influence.
Clone Trooper 1: Hey guys! There's totally a fight behind the hangar doors.
Clone Trooper 2: Let's go!
Clone Trooper 3: Fight! Fight! Fight!
Clone Trooper 4: I'm telling Lord Vad... oh wait.