You certainly can pull cards that you will to appear, which is why it is important that when you shuffle and do the pull that your mind is clear, without expectation (except for the expectation that you will receive the truth of whatever it is you are asking for). When I pull cards when my mind is desiring a certain outcome, I do often get skewed results. I know it too. I sometimes will even just put those cards back without even looking at them as I know it is just b.s..
I have known some people who can really throw off a reading, such as energy-workers who specifically create "glamours" and things like that. When you go to read them you then pick up on the glamour that they have created instead of the truth. Again, these are mostly energy-workers who are involved in a lot more dangerous things than tarot, and they are best avoided.
Besides that, it is possible that someone's will could affect your own when you are doing a reading for them, but that really depends on a lot of factors (such as how strong is their desire for a certain thing, how susceptible and open the reader is to influence, how clear is the reader in getting at the truth (or how much do they really just want the querent to be happy with the reading over finding the truth, which would then open them to a more colored draw and interpretation) etc.)
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