Any experienced, professional performer will tell you the only way you can appear in front of people looking unrehearsed is to practice-practice-PRACTICE.
Otherwise the uh’s and awkward pauses as one tries to remember their next line or action come across as amateurish, whilst the audience becomes increasingly embarrassed, uncomfortable, and possibly even angry. (After all, it is an imposition on their time and intelligence!)
A friend and experienced theatre audience member called recently to ask about just this subject. They’d suffered through an hour of an excellent example of how lack of rehearsal affected a public presentation.
Assuming it had been a failed attempt to appear ‘natural, informal and friendly,’ they called and asked: Doesn’t it take practice to appear informal or unrehearsed?
This question from someone who’s attended workshops of top Broadway and international performers, directors, producers, playwrights, choreographers, etc. In other words, they already knew the answer.
This doesn’t mean that a professional, with years of experience behind them, cannot give an excellent ad hoc performance; however, one must remember they’ve often decades of performance experience to rely on. Amateurs have no such thing.
No matter what your level of experience, nothing substitutes for PRACTICE.
And yes, your audience DOES know the difference.