Did they land troops in vast numbers to quickly over run a defensive position? Or did they lay on the beach and wait for breaks in enemy fire to slowly advance? Did they forecast very heavy initial losses if Japanese artillery is just waiting for them to land?
Or did they lay on the beach and wait for breaks in enemy fire to slowly advance?
That's the last thing a seaborne invasion force wants to do - anywhere, ever. The beaches offer very little natural cover (artillery craters have been used to make foxholes prior to landings) but overall the defenders will have pre-positioned their weapons to provide interlocking (overlapping) fields of fire, making no place safe from defending fire.
Defensive artillery will have pre-planned fire patterns that the defenders observers can call in based on where attacking troops and their stuff bunch together to make a good target for some high explosives.
Every inch of that beach is going to be covered with a lot of lead and things that go boom.
An invading force needs to progress inland as soon as possible, to overrun the beach's defenses, and to get to ground that can offer cover and concealment.
You also have subsequent waves of landing craft delivering fresh troops and equipment. Defenders who have already landed need to make room.
Here are some quotes from http://www.iwojima.com/battle/battlec.htm:
...Unable to dig foxholes, they were sitting ducks for the hidden Japanese gunners.
...Heavy fire made it impossible to land men in an orderly manner. Confusion reigned on the beaches.
...Japanese gunners zeroed in on every inch of the landing beach. Blockhouses and pillboxes flanked the landing areas. Within, more heavy weapons stood ready to blast the attacking Marines. Machine guns criss-crossed the beaches with deadly interlocking fire. Rockets, anti-boat and anti-tank guns were also trained on the beaches.Every Marine, everywhere on the island was always in range of Japanese guns.
TL;DR: You never just lay around on the beach waiting - it's a good way to die.
The strategy was pretty simple - Move inland until the enemy fires at you then destroy them with naval and artillery support. As far as expected initial casualties - the disaster at Tarawa forced planners to expect more initial losses for subsequent operations but a change to Japanese defense strategy later in the war - drawing the enemy into a battle of attrition - resulted in less than expected casualties on the beach. The invasion of Okinawa resulted in very few initial losses.