But it is his JOB as an expert witness to testify under "stressful" circumstances. I think it is reasonable to expect an expert witness to be able to answer questions in a deposition.
Ah - I was confused about the term (not familiar how the legal system works there) - I thought it meant an expert as a witness by chance. If it's his job to sit in the proceedings my comment was really off.
I'm not completely clear on the role of expert witnesses either, I assumed it was someone who was an expert in the field occasionally called to be a witness but their day job is not to be a witness (despite getting paid for it).
There are people who make a career out of being an expert witness. For engineers it requires having a PE certificate at a minimum which is rare in most industries in the US. This allows the professional experts to command high fees.