When completely absorbed in a meaningful task, a friend or stranger passes by and says "Hello" or initiates a conversation. I now find myself not being as kind as I would like to be as my mind is still completely absorbed in the task at hand.
Meditation has helped improve my ability to focus like this and let external events just flow by, but from a Buddhist perspective, how can I practice my ability to "switch" and be more present in the interaction with the by-passer when I actually want to?
Mindfulness in daily activities can be very important to one's Buddhist practice. Meeting every and all social expectation isn't so important to one's practice. So to the extent possible, you may wish to remain mindfully absorbed in your task and not look for social cues that strangers want to talk to you. There is really no need to "be more present in the interaction with the passerby than I actually want to". You can let go of the feeling that you need to be available to everyone.
Of course this won't always work and sometimes it's a friend that's come to talk to you and you must stop what you're doing and respond. Noting your feelings ("annoyed" or "bored" or "anxious") without judging the feelings can help you remain mindful throughout the time you are talking.
If your friends drop by unexpectedly very often, they may not understand your need for time alone for your practice of mindfulness and meditation and the constant interruptions can make it hard for you to sustain your practice. Talking to them about this may be helpful especially since you are feeling "not as kind" as you'd like to be and that's not good for your mental state or the continuation of your friendship.
In other words, there is value to remaining mindfully absorbed at times and limiting the circumstances under which you even need to switch focus may be more beneficial than trying to learn a skillful way of switching focus quickly over and over through the day.
Your awareness and focus should be on arising and passing of phenomena pertaining to the 4 foundations of mindfullness (impermanence if you still cannot see arising and passing of phenomena) with strong equanimity towards sensations and maintaining your mind free from clinging and craving. It is not the task that you should be absorbed in but experiencing phenomena as they come and go. Also any of the 4 foundations of mindfullness you have to experience at the experiential level hence through sensations. E.g. if you are looking at your body posture then it should be the outline of sensations that you experience through your sense faculties that any conceptualization of the particular posture.
What ever task you do keep looking at phenomena / sensations / elements arising and passing equnimously then it is enough. You can even multi task.
