What Font should i use - article in smashing magazine

Found interesting article on font selection for graphic design jobs.

Reference:
Mayer, J. (2010) What Font Should I Use. [Online] Available at: http://www.smashingmagazine.com/2010/12/14/what-font-should-i-use-five-principles-for-choosing-and-using-typefaces/ [Viewed on 18/01/14]

Notes the initial approach students take when choosing suitable typeface for a project:
"Many of my beginning students go about picking a font as though they were searching for new music to listen to: they assess the personality of each face and look for something unique and distinctive that expresses their particular aesthetic taste, perspective and personal history. This approach is problematic, because it places too much importance on individuality."

On Sans Serif, groups together Geometric Sans, Grotesk and Realist as have simular properties.  On what these types are best used for:
"At their best, Geometric Sans are clear, objective, modern, universal; at their worst, cold, impersonal, boring. A classic Geometric Sans is like a beautifully designed airport: it’s impressive, modern and useful, but we have to think twice about whether or not we’d like to live there."

On Humaninst - it derives from handwriting, has the human approach.
"a Humanist font generally have more detail, less consistency, and frequently involve thinner and thicker stoke weights "

On Old Style, its been changes gradually but consistantly over centuries. used plates:
"Old Style faces are marked by little contrast between thick and thin (as the technical restrictions of the time didn’t allow for it), and the curved letter forms tend to tilt to the left (just as calligraphy tilts). "

Categorizing personality type of Slab Serifs into a neat covering description is hard as they vary such within this subcategory, representing opposing moods or audience or place:
" Slab Serifs are an outlier in the sense that they convey very specific — and yet often quite contradictory — associations" 

THEMES:
Contrast or Correspondence.

Principle is this- when designing a piece of work, mostly one typeface will suffice.  When adding another typeface, either go big or go home; either add something so different and obtuse that it contrasts and they compliment each other, or keep identical types so that the sit neatly together and dont distract from visual communication.  If pick second type that is similar but slightly different it draws the eyes attention to the tiny difference rather than fluidly reading it:
" This creates an uneasy visual relationship because it poses a question, even if we barely register it in on a conscious level — our mind asks the question of whether these two are the same or not, and that process of asking and wondering distracts us from simply viewing."

AVOID:
Using two types from the same sub category as the difference is not enought and distracts the eye.

ADVICE:
"But if we want some principle to guide our selection, it should be this: often, two typefaces work well together if they have one thing in common but are otherwise greatly different. 

Shared aspects include: x-height, weight, time period, chronological, same designer, for e.g.

Display typefaces, consider this advice:
"‘do not exceed recommended dosage"


Extra notes from commenters:

Optima has vertical instead of diagonal contrast and this makes it suit better to be known as a modern sub category typeface.

Karsten Luecke