Do characters in movies actually need to pull the hammer back on guns?

you're correct. The pulling back of the hammer is to portray intent. However, it also puts the gun on a hair-trigger (a notch no wider than a hair - hence the name), so the gun is easier to fire. I fired a beretta 9mm at a range in Oz a few year ago. I shot 10 bullets straight, then the next 10 with the hammer back. the second ten had infinitely better accuracy