Aimed squarely at the lucrative US market, the Tracy family was based on a concept similar to the TV series Bonanza with Jeff Tracy even taking on an appearance similar to Lorne Greene. Jeff Tracy's sons were named after the first 5 American astronauts into space via the Mercury space project, Scott Carpenter, Virgil Grissom, Alan Shepard, Gordon Cooper and John Glenn (Gerry later wrote to the surviving astronauts with signed photos of their Thunderbirds counterparts, dryly stating , "Now you know why you became so famous").
Character visualisations were handled by Sylvia Anderson and the puppets were created by Chief Puppeteer Christine Glanville and her team. To give Scott and Virgil the appearance of being taller than the other members of their family the puppets were made with narrower waistlines. This technique was also used on the Hood who's puppet was fitted with larger hands and head to give the appearance of being more powerful. The puppets were all made to one-third life size (approximately 56cm high). Scott's appearance was based on Sean Connery, Alan Tracy's face was modelled on actor Robert Reed. Brains is thought to have been modelled on Anthony Perkin's. The puppet's again featured Gerry's electronic lip sync mechanism where the character's mouth movements were initiated by replaying the pre-recorded dialogue which sent an electric pulse from the control room to wires running alongside the puppeteers gantry. When a puppet was to be filmed speaking the puppeteer connected jumper wires to the puppet's control wires and thus connected it to the servo mounted inside the puppet's head or body.
The characters Lady Penelope and Parker were designed to suit the US's conception of the English class society, hence the exaggerated accents and appearance. When making the face for Lady Penelope, Christine developed Penny's looks from a model for a shampoo advertisement - Sylvia provided the voice for the character. Parker's character was based on a barman at the local pub the production team frequented. Arthur, the barman, had previously worked for 'er Majesty's Service and spoke with an accent that allowed free use of the letter 'H'.
Lady Penelope was a female James Bond, her family motto being "Elegance, Charm and Deadly Danger". This coincided with the production of a 007 movie at the same time as the screening of Thunderbirds. The Bond production crews often watched Thunderbirds on TV to check on the series and on special effects being made for the TV series. Derek Meddings later worked on five Bond films as SFX supervisor.
John Tracy was initially to be one of the main characters in the series but Gerry was never happy with his looks or voice and consigned him to Thunderbird 5 where he only made brief appearances.