What is the difference between the following introductory phrases?
To tell the truth
To be honest
Is any of them more old-fashioned or formal than the others or are they completely interchangeable?
Basically, they all mean in truth:
The last four are bit a bit more formal.
They are all the same when used in a facetious way.
Each of them connotes a different thing when used appropriately. As in:
To tell the truth,: You have misconstrued my words and started speaking at length on that basis. I could interrupt with 'To tell the truth, ...' to draw your attention to the fact that the truth is different from the presumption behind what you are currently saying.
...and so on.