I did my daughter's only last week. I have gone on loads of dwp assessments and not yet met a nasty assessor.
They will ask you to describe your condition - tell them - for instance if it makes you tired, say so, if it decreases your appetite say so, if the drugs you have to take make you "fuzzy" say so.
They will ask about your day - don't say I get up take the dog for a walk and then come back for breakfast - say, if it's a good day I get out of bed but sometimes it takes me a while to do this because... sometimes the dog doesn't get a walk because.... sometimes I can get my child dressed but sometimes I hve to phone for help because...
it's little things like if your condition means you are not strong tell them that you can hoover but you can only do the middle of the room you can't move furniture - if you aren't strong tell them that you can't carry shopping
obviously don't lie but tell them every little thing
for instance for my duaghter we said that she wakes up and she then has to transfer into her chair, it takes ages to do this and she has to try to scoop her little baby into her arms and then guide her toddler to the lift - if she didn't have a lift in the house she would have to sleep downstairs, then into the kitchen to get them breakfast but she can only do this becuase her kitchen has been adapted - even the door has been widened to get the wheelchair in and her carer has to come and help anyway... etc etc.
they have a points system and they give so many points for each thing - you have to get 12 points for each section - there are guides for the points online.
hth