Cognac is for Drinking, not for Dipping Your Cigar
In older movies, actors sometimes dip a cigar into a glass of cognac or other liquor. They seem to do it before lighting the cigar and even while smoking it. In imitation, rookie aficionados sometimes do the same but you won’t catch experienced cigar smokers dipping a prized Casa Magna or Padron into any sort of spirits. They might accompany their cigar with a high quality drink but that’s as close as they get to mixing cigars with cognac.
There’s a good reason for not dipping your cigar into a highball of cognac. Basically, it affects the flavor of the cigar. As nice as that cognac tastes, you don’t want to give your cigar the same flavor. This defeats the purpose of relishing the complex flavors that cigars impart. It’s not needed and is akin to dipping your cigars into beer, vanilla flavoring, or even orange juice. You wouldn’t think of ruining your cigar with those liquids so don’t do it with cognac either.
So why is it depicted in old movies? In all likelihood, the director probably thought it added an interesting dimension to the scene. However, back in the day, some cigar smokers also actually did dip their smokes into cognac and other liquors to moisten them before smoking. Use a humidor, though, and you won’t need to moisten your smokes with cognac or any other liquid!