Can I use these interchangeably?
I'd be lying if I said I wasn't a little surprised.
I'd be lying if I didn't say I was a little surprised.

Question

Answer

The first one is much more common. The second sounds strange to me, if you don't say something then you are not lying?

But there are no real problems with either. They both have the same meaning. I would use the first one.

Answer

They are similar but the first would be more proper and easier to understand.