-Natalie Goldberg, in Writing down the Bones
Natalie is describing how a writer, in a restaurant scene, can mention a fly, even the particular sandwich it walks over, name its species, but if she digresses too much in discussing the patterns of its wings and a mathematical discussion of its trajectory and such details unconnected to the scene where the reader is expecting the waiter to arrive or something else related to happen, how it can be a put-off.
While I believe that digressing is a fine art that is not very well explored yet - at least to my knowledge - let us leave it to another time.
The reason I thought of bringing in the fly in the ointment is its inappropriateness elsewhere. It is analogous to the mindless use of analogies. Suppose a great unnamed spiritual leader - or one treated as such - is describing the theory of unity of souls and how there is one big soul and all our souls merge with it once we die (or may be are one even when we are alive). A typical example he would take (I am giving up my political correctness by not saying she here. Howmuchever I would like to see the equality of sexes in the arena of priesthood, the males have unabashedly taken control over it, and the women should really do something abut it. But they should replace some rather than add to them) is to say that JUST LIKE sugar dissolves in to water and becomes invisible, unidentifiable, so also our soul becomes one with the ultimate one. This is an analogy. The sugar and water are tangible substances and have nothing whatsoever to do with the souls. Their physical properties in this case are based on the intermolecular spaces as well as the structure, composition of the molecules. But in one stroke that is equated with the soul which can otherwise not be decomposed in to any structure. When others seem to believe it, the guru starts believing it himself (in case he did not earlier), and all this has a very positive effect and it becomes a fact (just like ganesh idols drinking milk is a fact). Oh, the power of analogy. Poor Einstein, he once said: Make things as simple as possible, but not simpler. If only he could use analogies, everything would so much simpler.
PS: budding authors may find some good pointers in Natalie Goldberg's book.
Her website is: http://www.nataliegoldberg.com