I've run into this a couple times and was always curious. In what instances might you call someone by both their surname and given name? Ex. In a conversation, person B, refers to person A (named Yamada Tarou) as Yamada Tarou when speaking to them. Is this informal, formal, or rude?
This typically happens when you have two or more people with the same surname in your organization and they cannot be distinguished even with the suffixes such as -さん, -先輩, -課長 and -先生. If there are two 山田課長 in your office, it's fine to include their given name (e.g., 山田太郎課長) as a last resort. (In casual workplaces we also hear 太郎課長, too.)
Much less commonly, a full name is occasionally used to imply a psychological distance. If someone suddenly started to say 山田太郎課長 even though there is no need to disambiguate, I probably feel they are upset about him. (I think English speakers do similar things to scold their children. It's in the same vein.)