McLuhan gets this a bit screwed up

In the first part of Understanding Media, McLuhan also stated that different media invite different degrees of participation on the part of a person who chooses to consume a medium. Some media, like the movies, were "hot"—that is, they enhance one single sense, in this case vision, in such a manner that a person does not need to exert much effort in filling in the details of a movie image. McLuhan contrasted this with "cool" TV, which he claimed requires more effort on the part of the viewer to determine meaning, and comics, which due to their minimal presentation of visual detail require a high degree of effort to fill in details that the cartoonist may have intended to portray. A movie is thus said by McLuhan to be "hot", intensifying one single sense "high definition", demanding a viewer's attention, and a comic book to be "cool" and "low definition", requiring much more conscious participation by the reader to extract value. Wiki

Maybe he just doesn't complete the theory. Hot media do require attention because the media is well encoded. Movies are well designed to utilize as much of the media as possible, the sound track, the background and the character actions. Hence movies are well encoded, and the user must utilize a complete decoder in his head to get the complete picture.

Television is highly redundant, the director employing redundant cues to emphasize a simple character. High bandwidth, low encoding efficiency, simple repeatable messages. This technique allowed the weekly series.

McLuhan skips the two parameters that define the media, bandwidth and encoding efficiency. Media that rely on user interpretation are low efficiency encoders, television. Low encoding efficiency is relaxing, the user can miss part of the plot and still figure out the plot. One cannot do that with movies.

Broadcast radio quickly developed into the popular music station, frequent short songs and a fast speaking DJ. They had limited bandwidth and filled it with short information quants and only a mild underlying common theme. High efficiency encoding of a low bandwidth media.

The deal about McLuhan, television and tribalism was about the television industry trying to isolate groups with a known built in decoder, the demographic partitions. It allows them to encode directly to the selected group, making a simpler encoder tailor made to appeal to the selected audience.

McLuhan didn't quite get Shannon, but he was close.