The explicit use of interfaces is something I really try to follow and encourage others to start using.
Implement Interface Explicity
The second option for implementing interfaces implements the interface member effectively as a guarded public method, accessible solely through an interface reference instead of a public member. I don't think I've come across a development team that has used this option, but after experimenting with it, I think it's something that should be adopted a bit more by default.
My goal when writing code is to define contracts for the interaction between distinct units of work. This gives me the flexibility to swap out classes easily and quickly. This means I'm predominantly working through interfaces. The most significant benefit of implementing interfaces explicitly is that it means any change to the interface is immediately picked up in the implementing member(s) by the compiler. This means if I re-factor code and remove methods from an interface, the compiler notifies me on the next build that I now have dead code in implementing members.
By designing concrete classes with explicit interfaces, the public API for those classes are kept tidy, and it helps enforce usage through the contract interfaces. I hate seeing code "twisted" by having concrete references constructed and passed around. It defeats the point of defining an interface in the first place.