Many new improvisers are stumped by “create a character”, and feel that some kind of detailed backstory is necessary before they can say anything. The reality is that most compelling characters are defined by fairly concrete, accessible decisions. Choose things you can easily see and understand: where we are, what’s just happened and what I need right now. If I’ve sprinted across town, I should still feel out of breath. If I’ve been waiting for 3 hours, I’ll be a little numb. By focusing on physical sensations, you avoid getting caught up in the dangerous guessing that leads to an unnaturalistic performance.
And then there’s the issue of the voice. Overthinking will lead to made-up accents and a low pitched voice to indicate a shift, but this can get in the way of clarity and credibility. Work on the cadence and articulation instead. A nervous character might speak slowly and deliberately, whereas an excitable character will just start rambling. Record the same piece three times, first at a snails pace, then fast, and then as fast as humanly possible, and note the difference in character without a single word being altered. This test should show you that personality can be built in what you do, not what you decorate.
Furthermore, internal imagery is a more powerful tool for performance than textual analysis. Determine one thing the character is looking at that they value prior to performance—a picture in their pocket, a fight they had that morning, a vow they must keep—and carry it in your mind as you perform. The language you speak has resonance because it is tied to something specific, even if the audience does not know what that is. Often an actor’s attempt at too many pieces of internal imagery will diffuse the energy of their performance. One thing you can play is usually more effective than twelve things you cannot.
That is where a daily regimen comes in. Each day, for example, pick one physical attribute, one pacing choice, and one inner picture. Do the scene once, and then go back and change one of those choices. I recommend making a recording so you can listen back and decide which one feels more real and less showy. With time, you will get to a place where you can shift around like that and still be in character.
If it’s all getting a little ‘actory’, then go back to neutral. Just stand there, say the words, and have your real responses. Then you can add one choice to that and see how that goes. Acting isn’t sculpting something from clay, it’s turning up the lights on a sculpture that’s already there. Little tweaks can show you the contours of what’s already there, and that can make it real, without needing to invent much.