Philippians 2:12  Wherefore, my beloved, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling.
Does fear and trembling have to do with being so afraid that one physically trembles? Is that the sense in the Philippians passage?
To understand the sense of these words, let us look at two other passages, one from the Psalms, and the other from Paul's first lletter to the Corinthians.
Psalms 2:11  Serve the LORD with fear, and rejoice with trembling.
One cannot rejoice when scared to the point of trembling can one?
But what is this trembling allll about?
1 Corinthians 2:3  And I was with you in weakness, and in fear, and in much trembling.
Let's look at the context here:
 Corinthians 2:2  For I determined not to know any thing among you, save Jesus Christ, and him crucified.
3  And I was with you in weakness, and in fear, and in much trembling.
4  And my speech and my preaching was not with enticing words of man's wisdom, but in demonstration of the Spirit and of power:
5  That your faith should not stand in the wisdom of men, but in the power of God.
Paul was actually ministering with faith which released the "power of God", so that the faith of the Corinthians would not be based on his confident eloquence, but on the demonstration of the power of God.
An apostle demonstrating the power of God would hardly be running scared , so what did Paul mean?
Surely he meant that he was not relying at all on himself, his eloquence, but, in full distrust of his own abilities, he was relying only on the power of God
These days it is the fashion in human psychology to talk about "total self-confidence". But Paul was operating in total self-distrust.
Now transfer this insight to the Philippians passage. Surely Paul was saying tha we are to work out our salvaion iin total self-distrust, recognizing our inabiliyand relying only on the grace of God which would work in us to will and to do of God's good plesure. Self-reliance and self-confidence results in fleshly works.
"Fear and trembling" are the keys to being humble while being bold.