Depending on what court you are in, different levels of disclosure are required. Generally, you should be given notice of every witness the other side intends to call and a chance to question the witness. I recommend contacting a lawyer. The person who represents himself has a fool as a client.
Not enough info. You dont say whether this a motion or trial, what type of motion, what court, etc. If this is a motion, it would depend on the type of motion. There may be ways to challenge the affidavit. If someone shows up at a hearing with such a document, it is hearsay, and the court generally wont consider it, or will give a continuance for you to look into the issue. If it is small claims court, such affidavits are normally considered and permitted.
What sort of case? What sort of affidavit? Your question is a little vague, but generally, an affidavit by itself is not admissible evidence without the author or someone else to authenticate it. You are generally entitled to know the name of all witnesses as well as have a basic idea of what their testimony is expected to be. It appears you are representing yourself. Very bad idea.
Good luck to you.
An affidavit from a witness is only relevant, prior to trial, if it is being attached to a motion seeking some type of relief. It is not likely that the affidavit of a "witness" can be introduced at trial in lieu of that person actually taking the stand and testifying, as everyone is generally entitled to cross examine their "accusers".