I'm not a professional by any means, but I've done about a dozen musicals, and my advice to your tremolo question would be: 'slowly'.
If it's a high note (anything on the D or G string) you could probably do it relatively fast (and this will probably be quiet in most cases depending on what's going on on stage), if it's a low note (A or E string) go slooooow. Think of it more as unmeasured 16th notes rather than a true "tremolo". The cellist will be shaking like a leaf but in order to produce any amount of volume in low levels (and you usually want to be loud when doing tremolo on a low note) you need to make sure your grabbing the string with every stroke, so take your time and really pull out your sound.
That's what I do anyways
