The parties — the claimant and the respondent — will be asked to provide a written witness statement for every person who will give evidence to the tribunal, including the claimant.
Anyone who gives a written statement also needs to go to the hearing, so you should make sure your witness is happy to do this. If you want someone to be a witness but they’re not willing to go to the hearing you can ask the tribunal to order them to go. However this is usually not a good idea because you can’t be sure that what they say will be helpful for you.