Differences between "frank" and "honest"

I found a lot of people say "Frankly" or "To be frank" while the others say "Honestly" or "To be honest".

I know both of them mean that sb. is going to say sth. which is true in a direct manner. But, I keep thinking that if there is a difference between them and if yes what is it?

I am informed that in English as well as in many languages, some words are pretty same in meaning with each other, one of them is preferred by some people just because they like it. Like teenagers like to say some words which they think can make them cool. The case of "frank" and "honest", is it like this?

Here are some definitions:

Frank 1 : marked by free, forthright, and sincere expression - M-W
Honest 1 a : free from fraud or deception - M-W

Although frank has an element of honesty, its basic meaning is to be uninhibited in what is conveyed. That is, what the speaker says won't be filtered to cater to the listener's sensitivities.

Honesty can be expressed more circumspectly, but when someone starts a sentence with "Honestly, ..." you can expect a rather frank assessment.