Doug Stamper's niece and nephew are played by actor Michael Kelly's children: Franke Kelly and Clinton. When Franke hugs Doug and knocks him over the resemblance is obvious.
When Frank Underwood visits Petrov in the Jordan Valley, the Navy SEAL security team riding in the Hummers with him are all real Navy SEALS that Kevin Spacey worked with on Captain Philips. The Navy SEAL on his immediate left when he steps out of the vehicle is his actual bodyguard in real life.
Petrov's camouflage uniform is not Russian camo, but Danish. This is probably a hint to the fact that Lars Mikkelsen (Big brother of Mads Mikkelsen) is Danish.