I have done videos like this a few years ago, it was very educational and interesting to see some of the special issues involved. Your signer will already know to dress plainly, in something dark and without pattern, so the hands will be easy to see, but also keep the background they work in on the dark and flat side, don’t make it look like a backlit situation, make sure the lighting on the signer is suitably flat and doesn’t cast confusing shadows.
Remember that signing in real-time is a very physical activity, so you need to schedule breaks for resting the arms, or at least be observant of the signer and be ready to ask if they feel like taking a rest, if it’s going to be a long shoot.
As far as placing the window, I have seen upper right third/corner and lower corners, I have even see a 2-box, I think as much as anything it depends onthe avaialable technology. In case they later decide to add captioning, it’s probably best to keep the signer higher than the lower-third titling area where captions might someday go…
This one time I worked on a program for the deaf and had the deaf clients not only write and produce, but they were on the set, did much of the acting, and sat in on the start of the edit. (man, try to explain linear editing and tape pre-roll and the like in that situation, it was freaking them out, especially all the shuttling and a-b rolling, until I explained it a bit). Anyhow…
Comes the day to show the finished piece, all is well for a few seconds, then the clients are signing like a house afire back behind my chair. The same people that were in on every aspect of the program were now upset because the on-screen signing was done in Signed English and not ASL… by the client themself, who wrote and performed it!. I was off the hook because it was their mistake, and they accepted the blame for it. Lots of jokes were made on both sides about who’s not listening to whom. I think we re-shot a few inserts and let the rest go for deadline purposes. Overall it was a different but enjoyable experience, working thru the translator/signer with the clients and seeing their needs were met.
I bring it up so you know to ask in advance if there is a preference for the language choices made. Same as with Spanish tracks: do you want classic Castillian Spanish, PuertoRican Spanish, Cubano-Spanish, Mexican Spanish, Columbian…. there are many variations, and you want to make sure the client knows and approves the choices in advance.