Well, I've been a manager before and I can certainly help with this:
1 - I'd advise against a reference in writing unless you've worked with someone a long time. Even then, stick to the facts. What did they do for you, and how well did they do it. Don't make future statements. (ex. "I'm sure he will be great for you.") unless you have facts to back them up. The exception to this might be interns, you often have to submit a written report on the conclusion of an internship. At least I had to in IT.
2 - If you've only known them a short time, do talk over the phone but not in writing. Tell what you know, but never guess (positive or negative). Stay factual. For example, you can say you saw someone working on a light board, and they did well for your show. You can't say how good they are at lighting in general. If they were an intern, you can talk candidly about what you saw them do and, especially, how quickly they learned. As their supervisor, your position is to comment on their learning and improvement. (I've had to do this in other fields.) For interns, nobody expects them to be perfect, but people do want to know about attitude and learning. When working with interns in IT fields, I kept a checklist of the skills they demonstrated, and how those skills improved during their time. That helped to make these reference calls easier.
Hope that helps,
Chad