This time of year fish are likely to be much more aggressive. We already know they are very territorial but this is highly exacerbated as the reproductive urge builds in them. This is as a result of both hormonal changes and a desire to feed heavily to deal with the winter and for many fish the spawning period ahead. A big male trout is likely to savage a smaller trout that comes near his territory at this time. The big guy is gearing up for fertilising the hens so his instinct will be to keep others away.
There is also a lot of movement as fish migrate through river systems and you may find fish in water types and areas of pools that haven't held fish during the season.
In Ireland we will have larger lake run trout in our rivers, locally they have names such as Croneen and Dollaghan. Traditionally they are fished for with smaller wet flies and often in low water at night, however they are also very partial to a 4 inch plus articulated streamer! Both trout and salmon often wake up from their slumber where they may not have been active for some time. For the reasons above we may have a target that wasn't available during the summer.