Steve's new body was literally made for battle. It came naturally to him now, strength, stamina, agility, the whole shebang. He suspected the serum must have messed with his brain a little as well, since he could memorize a battle plan in just a few seconds, and easily break down an enemy's weak points. This all felt natural, instinctive, like this is what he was made for.
Steve's new body was not made for acting. At least not at first. It didn't help that they didn't really have a rehearsal either.
They talked him through it and it seemed easy enough:
"Walk down stage... Yes that means toward the audience"
And: "Say you're lines, blah blah blah"
Then: "Go to this mark say more lines blah blah"
And finally: "Turn and punch Fake Hitler, blah blahdy blah"
Beyond that he was given no further instruction. So of course, on the first night, he turned and punched Dave right in the nose. Apparently Dave was quite the actor because he immediately covered the dripping mess and crawled off the stage behind the girls.
Steve felt terrible, he visited Dave in the hospital and begged him to come back. Sadly, the damage done to his nose made him look distinctly un-Hitler and he had to move on to what was (to Steve's great relief and pride) a very well paid and prestigious role in theatre.
The next guy was not so lucky. Steve was given some basics on stage combat and then set loose on George. Unfortunately, Steve missed (and seriously, how do you miss the air) and broke Georges jaw. Fortunately, the audience thought Hitler laying dead still the rest of the show was planned and it got a good chuckle as he stayed there when the stage was emptied.
When Steve visited him in the hospital George cowered away from him in fear, which of course broke the poor Captain's heart. After he was released, George found a hefty sum deposited in his bank account that covered his expenses. Steve didn't really need the allowance he got from the Senator anyway.
From then on the scene was rehearsed very thoroughly step by step every night of the performance. The only other accident was when he kicked a hole in his dressing room door trying to get his damn tights on.