To put witness testimony into evidence at a trial, you must call the witness to the stand. They swear to tell the truth. Then, you have the chance to ask them questions.
When you’re done asking questions, the other side has a chance to ask them questions too. It’s not enough to submit an affidavit of the person’s statement because that doesn’t allow the other side the opportunity for cross-examination. In most cases, you need to bring the witness to testify during the trial.