This they do with impunity all the time. They are always out and about, and intermixing in human society. This allows them to do many things of strategic value. They can appear in the exact form of someone else, and then carry out some action in their name, and this will be completely undetectable by those present who will unfailingly assume it is the real person because the disguise is so perfect. These imposters do not have the full array of knowledge and personality characteristics downloaded from the actual process used to create a full-fledged imposter with elimination of the human themselves. So there is a limit to the deception that is possible, but when this is done for specific purposes, can be very effective in most cases.
There is infiltration to plant seeds, to sow discord, to stir up trouble, or to gather some direct intelligence, and any form that appears to be human will often suffice. They can assume the credentials and appearance of someone in government, military, or intelligence agencies, and gain access to secret meetings, and so forth, and only have to be careful to not be present when their real subject might also be there. This is taking some risk, but with the ability to do mind control manipulation, anyone who suspects something unusual in perhaps noticing there were a situation where someone seemed to be in two places at once, that individual can be manipulated to forget about this perception, and secrecy will be maintained readily. There can be slipups in the maintenance of human form. This depends on the skill of the individual Reptilian and their ability to focus on their mission and to not have their mind wander in a way that begins to shift the bodily instructions away from their temporary state. This is seen by observers noticing a change in surface appearance of areas of the body, for example, and this includes the appearance of the eyes as well, when they may suddenly take on a different appearance, for example, vertical pupils that are in keeping with the Reptilian form, or the scaly appearance of the skin.
You know of people who have observed such things in relatively commonplace settings. This reflects the fact that their disguise can slip at times, and also the fact they are commonly out and about throughout human society. In part, this is acclimation training, and gaining practice in doing human tasks. There needs to be familiarity with human customs, human technology, and behavior. So some of these are training exercises, some of them are in conjunction with maintaining a subterfuge over a period of time when there may need to be an arrangement to interact with others and do human errands and maintenance chores, and so forth. So they need to understand the culture and the physical environment of humans in a fairly detailed way for such missions.