Slitheen: The Infiltration

How do the Slitheen Infiltrait?
A good question... with a long answer.

Before the Slitheen Infiltrait a planet, they of course make their plan. They address all aspects: Who they need, who could stop it, who is possible of working it out.. etc. When they have done this, the Slitheen head for who they need. No, they don't hire this person: the Slitheen are brutal, they'll simply kill the person on the spot. If they are big enough, the Slitheen will turn the person into a skinsuit. They do this by clearing the insides, attaching a zip to the forehead so they can easily remask, and unmask. When the Slitheen have got Phase 1 complete, they then head for Phase 2. This is an example from Aliens of London.

Phase 1: Kill the Primeminister, then working out who would become it: Joseph Green so he was big enough to fit into so he was killed for his skin. Margaret Blaine, for the cover story of the Primeminister going to the car, and Oliver Charles for Transport Liason for the car disappearing.

Phase 2: When Oliver Charles (Slitheen inside the skinsuit) had completed his job, Joseph, Margaret and Oliver unmasked in order for the Slitheen inside of Oliver Charles to impersonate General Asquith. The group of 3 murdered him for control of the British Army. They hid the skin of Oliver Charles in a cupboard.

Phase 3: To kill anyone who might stop them in their plan, or figure it out. For this job, Sip Fel Fotch Pasameer-Day Slitheen was sent to eliminate Jackie Tyler. Whereas the Rup, the brother in disguise as Asquith unmasked as a distraction infront of the experts and the Doctor as a distraction. Jocrassa then killed the experts, but the Doctor managed to escape.

Phase 4: Finalise the plan, and eliminate any final people who are involved. Blon was already unmasked in the search of Rose & Harriet. Jocrassa and the brother unmasked in a lift, in order to help Blon. When they all arrived, it made the situation more tense for Rose & Harriet then showing themselves. Fortunatly the Doctor rushes in, and saves them.

Phase 5: Finalise EVERYTHING so the plan is secure. At points in their plan, the Slitheen were extremely close to being revealed, when they were unmasked so they quickly had to remask.

A short answer: If the Slitheen need a person as a part of their plan, they will kill them and convert their skin into a suit. Or, if you could be in the way of their plan, they will kill the person. The Slitheen kill anyone needed, or associated.

The only major thing for the Slitheen is not to be discovered, the major issue is all of the farting from the gas exchange caused by the compression device on their neck, but another thing  is when they are unmasked. Here we see the Slitheen unmasked, quickly remasking so it isn't discovered by the Police. This was a very close one for the Slitheen so it was lucky to avoid detection.