Back in the 50s and 60s, as OP so fondly remembers, and companies actually sponsored production of shows instead of just buying ad time through networks, stars would actually film commercials in character.
So you might see Clark Kent and Jimmy Olsen having breakfast together with Kellogg's Corn Flakes on the table while they talk about how great it is. It was okay for Clark and Jimmy to have breakfast together, but Lois wasn't allowed in those.
Or Andy Taylor and Opie chatting about Post Toasties, Lucy and Ricky Ricardo plugging Philip Morris cigarettes. Even the Cartwrights would ride up and tell you all about the new Chevrolets in a 10 minute plug every season when the new models came out.