That is a good question, why would live actors be required for SUPERMARIONATION? Other than close up shots of hands that had been frequently used in the past, The Secret Service saw great use of live actors in shots. The team had first been experimenting with live-action shots in Thunderbird 6 (1968), featuring shots with the Tiger Moth, or rather Thunderbird 6 itself. Examples included towards the end of the film where the plane uncontrollably flies along the M104 Motorway before crashing into a tree. These shots actually featured shots along the newly constructed M40, some shots could be filmed in the puppet studios though. Live-action shots were once again used in Joe 90's The Unorthodox Shepard when the team went on filming location at the church. With the team proving live-action shooting to be popular, it was therefore decided to use this technique for the next show.
Very often the puppet team would go out and film shots with the real actors, either actor dressed in the character's clothing, Stanley Unwin appeared in his own shots. Many other live-action shots were the cars themselves, particularly Gabriel who would appear in the UFO episodes Identified and A Question Of Priorities. With this becoming more and more popular of filming on location, it obviously meant less shooting for puppets. The only time we really only saw puppets being used was for longer range shots and interior locations. Anyway, the puppet shows were simply becoming more and more expensive to make with the endless reuse of puppets that was available. The Secret Service is now regarded as a hybrid series where uses many live-action shots with puppets in other areas. This shows the development of how the shows were beginning to morph from puppetry to live-action, Gerry Anderson had always wanted to make live-action and wanted always to achieve the best he could with his puppets to make them most realistic convincing.
This could be a reason why he once said The Secret Service was his favourite of all his shows, being the closest of what he would always want to achieve.