While it is unlikely (but not impossible) that your own PCs will ever need to enter the interior of a Dalek it is still useful to know more about the species. It can provide valuable information and maybe suggest sources of vulnerability.
Firstly the eyestalk is hollow, allowing the shrunken medical team to enter the Dalek. There appears to be a barrier that creates strange distortions as the Doctor and Clara enter, warping both time and space. This could be some form of forcefield that prevents small objects (such as dust particles) from entering the stalk.
Once past this area the group are able to proceed along the eyestalk, pulsing lights represent visual impulses, heading towards the brain. What appears to be dry ice can be seen on the floor (and was also visible swirling in the distortion field). This gas could cool the stalk or possibly act as a decontaminant.
This leads to the cranial ledge, located in the upper dome of the Dalek’s head. This allows them to look down upon the Kaled mutant, linked to the machine through wires and tentacles.
Also located within the cranial ledge is a cortex vault. This is an electronic brain, serving as a memory bank. Further more, by repressing certain memories it maintains the Dalek’s hate.
This proves to be a major plot point for the episode. This Dalek saw a star being born and had an epiphany that life would always return. Once repaired the cortex suppresses that memory, returning it to its old ways.
We can presume that this Dalek isn’t unique and others of its kind have similar experiences that would otherwise sway it from its mission to eradicate all life. Targeting the Cortex vault might therefore be the greatest vulnerability of a Dalek.
Preventing it from suppressing the memories would cause other Daleks to gain freewill, allowing their experiences to influence their decisions. Taking this a step further new memories could be implanted, effectively brainwashing the Dalek.
The Cortex vault could be reprogrammed to suppress violent or hate filled memories, making the Dalek a productive and peaceful member of society. Some could take advantage of this, abusing the Dalek knowing it would forget soon after.
This is probably how the Daleks were made to forget the Doctor, their cortex vault suppressing their memories of him. Could such a feat be achieved for a whole species? For example could the Daleks be made to forget humanity?
The soldier Ross learns to his peril that the interior of the Dalek is very sensitive. By firing a grappling hook into the metal it causes the Dalek pain and despatches floating orbs that act as antibodies.
The orbs are equipped with a powerful disintegration beam, that turn Ross to dust in seconds. It then sucks the remains up (its blue light turning red upon completion) and deposits the remains in an organic disposal unit.
Due to their small size it is unlikely that they could do much harm to a full sized human but cause some irritation. This might be a nasty surprise for those who are able to destroy a Dalek, only for a swarm of antibodies to attack them with their tiny disintegration beams.
The organic disposal unit contains green fluid. The Doctor explains that a Dalek needs protean and will harvest it from victims. It isn’t clear how it does this, as unlike the antibodies beam, their death ray leaves victims intact. Possibly the Daleks will convert dead bodies but we’ve never seen this on screen and presumably they could get protean from any organic source. It could be that they are conditioned to eat people to motivate them to kill.
Decontamination tubes lead from the organic disposal unit, which can get hot. This presumably prevents infection spreading through the rest of the Dalek interior. Following these tubes takes the group into the lower section of the Dalek (its skirt).
After passing through some circuitry they reach its trionic power cells. A breach in this cell releases near lethal levels of radiation. Destroying a Dalek, and thus rupturing these cells, could expose areas to similar dangerous levels of radiation.
In the centre of the Dalek is the mutant. Wired into the rest of the machine the mutant is presented with a holographic screen. This allows it to see not only the exterior but observe events within its own body.
It is also possible to display memories on these holographic screens as Clara demonstrates when she unsuppresses one of its memories. Potentially it might be possible to make a Dalek believe that a memory is happening in the present.
By crossing some wires the Doctor is able to share his own memories with the Dalek. While the Doctor is doing this willingly the Daleks themselves might use a similar technique to gather information from prisoners. Certainly this demonstrates they have the technology to link their own minds with others via wires (whether they would permit themselves to be contaminanted in this way is another question).
Filled with the Doctor’s hatred Rusty the Dalek almost single handily wipes out the invading Daleks. This shows that their shell isn’t resistant to their own weapons. The capture of Dalek weapons could therefore give rebels a great advantage.
Finally the Daleks appear surprised to be attacked by their own kind. This would indicate that it has been long time since there has been any division amongst the Daleks. If rebels aren’t able to convert a Dalek then they might still gain a tactical advantage by disguising themselves as one, just so they can take them by surprise.
These details of the Dalek interior only apply to the current model. In theory earlier models might be quite different, with the features seen here developed over time. Whether that is the case of not the Doctor is an obvious expert, able to identify every part they see.